Studies facilitate more effective weed control

Dustin Hensley, postdoctoral student in weed science, at left, and Dr. Eric Webster, LSU AgCenter weed scientist, operate a research combine in a weed test plot at the south farm of the Rice Research Station. The combine was purchased with funds from the Louisiana Rice Research Board.

Studies funded by the Louisiana Rice Research Board allow LSU AgCenter scientists to recommend strategies that facilitate more effective weed control for the state’s rice farmers.

"We keep harping on this, but you’ve got to get Newpath out early," he said. "If you wait until two- to three-leaf rice, your weeds are going to be that big, too, and you’d better have another herbicide with it."

Webster advised using the full 6-ounce rate, instead of 4 ounces, for the first Newpath application, which he said should be applied when the rice first emerges.

The LSU AgCenter weed scientist said it’s the earlier weed growth that hurts profits at harvest. "Most of your yield loss due to weeds is in your first three weeks after emergence," he explained.

Webster also warned cutting rates can be penny-wise but pound-foolish.

He said research associate Tyler Carlson has done research that indicates using propanil with Newpath is extra insurance against profit loss from weeds. "Your yields will be greater and your returns are higher," he said.

Webster said letting weeds get out of hand one year only leads to even greater problems for future growing seasons.

"If you don’t control weeds this year, you’re building the seed bank for next year," he said. "To maximize profits, you have to be aggressive on your weed control."

Webster said some resistant weeds are showing up in fields and the cause usually can be traced to growing Clearfield rice with no rotation.

He said the possibility of resistance also can be reduced by varying herbicides with different modes of action, such as alternating Newpath with Clearpath and also using propanil.

Webster said no new herbicides are expected to be released in the near future, even though material with new names will appear on the market. "A lot of our new herbicides are the old herbicides that have been repackaged," he explained.

Dr. Bill Williams, LSU AgCenter weed scientist at the Northeast Research Station, said north Louisiana farmers sometimes improperly identify sprangletop as barnyard grass. At an early stage, he said, the two weeds are difficult to distinguish, but sprangletop, unlike barnyard grass, has a ligule, a small membrane found where the leaf attaches to a plant’s main stem.

Williams said farmers will get improved grass and red rice control if Prowl is used with the first application of Newpath.

He said grasses in fields of conventional rice varieties can be treated effectively with an early application of propanil mixed with Prowl, Facet or Command.

Williams said farmers have complained Clincher and Ricestar are not performing well against grasses and that the problem usually is late application. He said some farmers gauge their application timing by the height of grass but that grasses should be sprayed before developing multiple tillers.

The LSU AgCenter weed scientist said research is showing farmers are overlooking the yield losses caused by broadleaf weeds and sedges, choosing instead to be more concerned about grasses.

"These isolated problems can become widespread if the problem isn’t controlled early," Williams said. "By the time I get called out to look at a problem, it’s often too late."

Checkoff funds for these projects in 2010: $153,500

(This article was published in the 2011 Louisiana Rice Research Board Annual Report.)